Completion of error handling

[elfring]

Would you like to add more error handling for return values from functions like the following?

• fputs ⇒ write_line
• pthread_key_create ⇒ Thread::init

[diovudau]

Thank you for your contribution. I am sorry for the late reply and I have yet to review this. But if these turn out to be no-brainers any improvement is very welcome.